Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together metal workpieces during the manufacture of a vehicle door, hood, trunk lid, lift gate, and/or body structures such as body sides and cross-members, among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly composed metal workpieces—such as steel-to-steel and aluminum-to-aluminum—the desire to incorporate lighter weight materials into a vehicle body structure has generated interest in joining steel workpieces to aluminum workpieces by resistance spot welding. The aforementioned desire to resistance spot weld dissimilar metal workpieces is not unique to the automotive industry; indeed, it extends to other industries that may utilize spot welding as a joining process including the aviation, maritime, railway, and building construction industries, among others.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through overlapping metal workpieces and across their faying interface(s) to generate heat. To carry out such a welding process, a set of opposed spot welding electrodes is clamped at aligned spots on opposite sides of the workpiece stack-up, which typically includes two or three metal workpieces arranged in lapped configuration, at a predetermined weld site. An electrical current is then passed through the metal workpieces from one welding electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface(s). When the workpiece stack-up includes a steel workpiece and an adjacent overlapping aluminum workpiece, the heat generated at the faying interface and within the bulk material of those dissimilar metal workpieces initiates and grows a molten aluminum weld pool that extends into the aluminum workpiece from the faying interface. This molten aluminum weld pool wets the adjacent faying surface of the steel workpiece and, upon cessation of the current flow, solidifies into a weld joint that bonds the two workpieces together.
In practice, however, spot welding a steel workpiece to an aluminum workpiece is challenging since a number of characteristics of those two metals can adversely affect the strength—most notably the peel strength—of the weld joint. For one, the aluminum workpiece usually contains one or more mechanically tough, electrically insulating, and self-healing refractory oxide layers on its surface. The oxide layer(s) are typically comprised of aluminum oxides, but may include other metal oxide compounds as well, including magnesium oxides when the aluminum workpiece is composed of a magnesium-containing aluminum alloy. As a result of their physical properties, the refractory oxide layer(s) have a tendency to remain intact at the faying interface where they can hinder the ability of the molten aluminum weld pool to wet the steel workpiece and also provide a source of near-interface defects within the growing weld pool. The insulating nature of the surface oxide layer(s) also raises the electrical contact resistance of the aluminum workpiece—namely, at its faying surface and at its electrode contact point—making it difficult to effectively control and concentrate heat within the aluminum workpiece. Efforts have been made in the past to remove the oxide layer(s) from the aluminum workpiece prior to spot welding. Such removal practices can be impractical, though, since the oxide layer(s) have the ability to regenerate in the presence of oxygen, especially with the application of heat from spot welding operations.
Moreover, in many instances, the persistence and tenacity of the oxide layer(s) are enhanced when an intermediate organic material layer—such as an uncured yet heat-curable adhesive, a sealer, a sound-proofing layer, etc.—is present between the steel and aluminum workpieces at the faying interface. This phenomenon is believed to be caused by residual organic material that is left behind at the weld site after the bulk of the organic material layer is laterally displaced along the faying interface by the clamping pressure of the welding electrodes. The residual organic material is then burned off at the location of the weld joint during current flow. It is believed that carbon residues such as carbon ash, filler particles such as silica and rubber particles, and other derivative materials end up protecting the oxide layer(s) against mechanical break down and dispersion during current flow, thus resulting in oxide layer remnants remaining grouped and compiled at and along the faying interface in a much more disruptive manner as compared to instances in which an intermediate organic material layer is not present between the steel and aluminum workpieces.
In addition to the challenges presented by the one or more oxide layers contained on the aluminum workpiece surfaces, the steel workpiece and the aluminum workpiece also possess different properties that tend to complicate the spot welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and relatively high electrical and thermal resistivities, while aluminum has a relatively low melting point (˜600° C.) and relatively low electrical and thermal resistivities. As a result of these physical differences, most of the heat is generated within the steel workpiece during current flow. This heat imbalance sets up a temperature gradient between the steel workpiece (higher temperature) and the aluminum workpiece (lower temperature) that initiates rapid melting of the aluminum workpiece. The combination of the temperature gradient created during current flow and the high thermal conductivity of the aluminum workpiece means that, immediately after the electrical current ceases, a situation occurs where heat is not disseminated symmetrically from the weld site. Instead, heat is conducted from the hotter steel workpiece through the aluminum workpiece towards the welding electrode on the other side of the aluminum workpiece, which creates a steep thermal gradient in that direction.
The development of a steep thermal gradient between the steel workpiece and the welding electrode on the other side of the aluminum workpiece is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum workpiece after the flow of electrical current has ceased, the molten aluminum weld pool solidifies directionally, starting from the region nearest the colder welding electrode (often water cooled) associated with the aluminum workpiece and propagating towards the faying interface. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, micro-cracking, and surface oxide residue—towards and along the faying interface within the weld joint. Second, the sustained elevated temperature in the steel workpiece promotes the growth of brittle Fe—Al intermetallic compounds at and along the faying interface. The intermetallic compounds tend to form thin reaction layers that are generally considered part of the weld joint. Having a dispersion of weld defects together with excessive growth of Fe—Al intermetallic compounds along the faying interface tends to reduce the peel strength of the weld joint.
In light of the aforementioned challenges, previous efforts to spot weld a steel workpiece and an aluminum workpiece have employed a weld schedule that specifies higher currents, longer weld times, or both (as compared to spot welding steel-to-steel), in order to try and obtain a reasonable weld bond area. Such efforts have been largely unsuccessful in a manufacturing setting and have a tendency to damage the welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical fasteners such as self-piercing rivets and flow-drill screws have predominantly been used instead. Such mechanical fasteners, however, take longer to put in place and have high consumable costs compared to spot welding. They also add weight to the vehicle body structure—weight that is avoided when joining is accomplished by way of spot welding—that offsets some of the weight savings attained through the use of aluminum workpieces in the first place. Advancements in spot welding that would make the process more capable of joining steel and aluminum workpieces would thus be a welcome addition to the art.